Turbulent skin friction active flow control: retrospectives, possibilities and open questions
Jacopo Serpieri
Abstract: In the last decades, to overcome the relatively small drag reduction levels achievable with passive approaches, mainly based on geometric features or dynamic properties of the flow-wetted surfaces, active flow control efforts have boomed in the fluid dynamics community. Several approaches have been explored by many groups around the world. Starting from the more traditional wall blowing and oscillating wall along the spanwise direction and proceeding to more advanced ones as modulating the spanwise oscillation along up-/down- stream traveling waves, or waves propagating along the spanwise direction, if not even waves of wall-normal motions.
While the major bulk of the literature studies exploited numerical simulations and usually considered the computationally more affordable channel flow configuration, experimental efforts, considering setups of fascinating complexity, have also given important confirmations and novel inputs. While, for the experimental efforts, the flow control systems are challenging to design, fabricate and operate, these endeavors allowed to explore higher Reynolds number flows and developing external flows. Finally, to attempt to simplify the possible flow control embodiments, novel actuators have been proposed and tested. These mainly rely on the Electro-/Magneto- Hydrodynamics (EHD-MHD) principles which, on the other end, extended the range of knowledge fields involved beyond the boundaries of fluid dynamics.
Besides all these efforts, different major aspects of active skin friction control are still to be understood. These relate to the scalability of the proposed control to higher Reynolds number flows, to the different flow control approaches and mechanisms and to power budget considerations which are mandatory for active control efforts.
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